Process and plant for evaporative concentration and crystallization of a viscous lactose-containing aqueous liquid

ABSTRACT

Crystalline α-lactose monohydrate is recovered frown a viscous lactose-containing aqueous liquid by subjecting said liquid to simultaneous heating, removal of evaporated vapor and mechanical agitation at high shear rate to provide a crystallization promoting decrease of the viscosity of the liquid with crystals formed and suspended therein to progressively concentrate the agitated liquid and simultaneously crystallize lactose therefrom. Subsequent cooling, drying, and disintegration yield particulate α-lactose monohydrate.

FIELD OF THE INVENTION

[0001] The present invention relates to the recovery of lactose fromliquids formed as by-products in the dairy industry and relatedindustries.

[0002] More specificly, the invention relates to a process and a plantfor converting lactose-containing liquids into easy to handleparticulate products having non-caking and free-flowing characteristics.

[0003] In the present specification and the attached claims the term“lactose-containing liquids” is used as comprising solutions, whereinall lactose is in a dissolved state, as well as slurries of lactosecrystals in lactose solutions.

BACKGROUND OF THE INVENTION AND PRIOR ART

[0004] Lactose is a disaccharide, which in solid state exists in atleast three forms, viz. α-lactose, crystallizing as the monohydrate fromaqueous solutions at temperatures below 93.5° C., β-lactose, andamorphous lactose.

[0005] Of these three forms, the α-lactose monohydrate is preferredsince it when relatively pure forms stable, hard, and non-hygroscopiccrystals.

[0006] In contrast thereto, especially the amorphous form of lactose ishygroscopic, and the presence of even minor amounts of the amorphousform on crystals of α-lactose monohydrate may impair the non-caking andfree-flowing characteristics of the latter.

[0007] Lactose is present in a concentration of approximately 5% in cowmilk, which is the dominating source therefore.

[0008] Nearly all cheese-making processes have a by-product, whey, whichis an aqueous solution, which, besides nearly all the lactose originallypresent in the milk, contains some fat, protein, and inorganic salts.The term “whey” is here applied in a broad sense, comprising cheesewhey, rennet casein whey, acid whey, and salty whey.

[0009] With the primary purpose of recovering protein values from thewhey or from milk, this is often subjected to an ultra-filtration,whereby the proteins are obtained as retentate, whereas the lactose andmost of the minerals remain dissolved in the water passing through thefilter membrane. This aqueous solution of lactose with highly reducedprotein consent is simply termed “permeate”. This is the lactose sourcepreferred in connection with the present invention although otherwhey-related products come into consideration.

[0010] Before used as starting material in the exploitation of thepresent invention, such permeate or related whey products may besubjected to a purification, e.g. a demineralization, and it is,preferably by means of vacuum evaporation, concentrated to a dry solidscontent of 40-75% by weight.

[0011] The main applications for lactose are as sweetener for ice creammixes, for baking applications, and as component of animal feed butsubstantial amounts are also used as nutrient in culture media formicro-organisms in the bio-technical industries. For the baking industryit is, inter alia used with the purpose of obtaining a desired browncolour of the bread crust.

[0012] Although these various fields of application have differentrequirements as to purity and visual appearance of the lactose, it isfor all applications desired to use the lactose as a particulatenon-sticking, non-hygroscopic, and non-caking free-flowing product.

[0013] Several processes have been suggested and used for convertinglactose in whey or whey-derive products into a particulate material.

[0014] However, a relatively fast drying process as obtained by spraydrying results in particles, in which only a part of the lactose ispresent as α-lactose monohydrate, whereas the remaining part thereof ispresent in a form which makes the particles sticky and hygroscopic.

[0015] Thus, a certain rest time is necessary before the cooled productresulting from the drying process can form free-flowing particles.

[0016] However, even after such treatment the resulting particles showtendency of hygroscopicity and caking.

[0017] This principle of fast drying combined with a subsequent rest isutilized in the FILTERMAT® process (Niro A/S) disclosed in U.S. Pat. No.4,351,849 and also in the process disclosed in U.S. Pat. No. 5,006,204(Assignee: A/S Niro Atomizer). In this last-mentioned process, apre-crystallization is used before the spray drying, and the spray driedmaterial rests on a rotating disc before after-drying in a fluidizedbed.

[0018] A different concept is used in U.S. Pat. No. 6,335,045, accordingto which the lactose-containing concentrate is heated to a temperatureabove the crystallization temperature and thereafter subjected to aflash process resulting in a concentrate of higher dry solids contentthan the starting concentrate, which concentrate is subsequently cooledto induce crystallization. Afterwards, the crystallized concentrate canbe dried in a spin-flash dryer. However, even with this rathercomplicated process, the results are not always satisfactory in terms ofcaking properties.

[0019] In the considerations preceding the present invention, we assumedthat if a higher percentage of the lactose in the particulate productwere present as the α-lactose monohydrate form, the properties in theabove-mentioned respects would be improved, and this turned out to becorrect.

[0020] Highly concentrated aqueous lactose solutions have highviscosity, and we assumed that this could be an important reason why thecrystallization of α-lactose monohydrate in the prior art processes hasbeen incomplete.

[0021] The invention is based on the recognition that it is possible byintensive agitation of the concentrate while it is being furtherconcentrated by evaporation, due to the pseudo-plastic and thixotropicproperties of the concentrate, to reduce the viscosity thereof to anextent, which highly promotes crystallization. The beneficial effect ofthis decrease of viscosity on crystallization more than compensates forany negative effect on the crystallization process caused by theintensive agitation. Thereby it is achieved that formation and growth ofcrystals occur simultaneously with the evaporation of the concentrate.Thereby, the viscosity is lowered, not only due to the pseudo-plasticand thixotropic characteristics of the concentrate mentioned but alsobecause an excessive viscosity increasing super-saturation would beprevented.

[0022] It should be observed that generally the effect of mechanicalagitation on the crystallization process is somewhat unpredictable.Reference is made to Mullin and Raven: “Influence of mechanicalagitation on the nucleation of some aqueous salt solutions”, Nature,Vol. 195, page 35-38 (1962). According to said paper, nucleation, whichis a prerequisite for crystallization, is generally higher in amoderately agitated liquid than under quiescent conditions but withincreasing intensity of agitation, nucleation decreased and at stillfurther intensities increased again and finally some decrease wasobserved. In said paper it is proposed that this unpredictable effect isa combined result of the influence of the agitation on the diffusion andthe attrition.

[0023] In most commercial crystallization processes where relativelylarge crystals are desired, such as in the sugar industry,crystallization is performed at rather gentle agitation of thecrystallizing medium, e.g. by using so-called crystallization cradles.An intensive agitation is regarded as harmful, not only due to attritionbut also because it may result in the formation of too manycrystallization embryos causing too small and uneven crystals.

[0024] However, as mentioned above, it has turned out that due to thepseudo-plastic and thixotropic properties of the lactose concentratesunder evaporation an improved crystallization result is obtained byusing a mechanical agitation of high shear rate.

SUMMARY OF THE INVENTION

[0025] The present invention thus relates to a method for evaporativeconcentration and crystallization of a viscous lactose-containingaqueous liquid, comprising:

[0026] (a) introducing the lactose-containing aqueous liquid having adry solids content of 40-75% by weight into a zone and thereinsubjecting it to the following simultaneous measures:

[0027] i) progressive heating at a temperature above 40° C. but belowthe maximum temperature for crystallization of α-lactose monohydratefrom the liquid;

[0028] (ii) removal of vapour evaporated from the liquid; and

[0029] (iii) mechanical agitation able to provide a crystallizationpromoting decrease of the viscosity of the liquid with crystals formedand suspended therein;

[0030] to progressively concentrate and simultaneously crystallize theagitated liquid, and

[0031] (b) recovering the resulting slurry from said zone at a totalsolids content above 75% for cooling and optional disintegration andfurther drying.

[0032] By this process, optimal conditions for crystallization ofα-lactose monohydrate are achieved.

[0033] The mechanical agitation drastically decreases the viscosity ofthe liquid being evaporated, whereby lactose molecules in the onlyslightly super-saturated liquid easily diffuse to the surface ofcrystals and nuclei therefore to promote crystal growth meaning that thecontent of dissolved lactose and thus the degree of super-saturation ofthe liquid decreases.

[0034] Also the simultaneous progressive evaporation and crystallizationensures the existence of optimal conditions for crystallization over aprolonged period and over a broad range of temperature levels of theliquid.

[0035] The above measure “removal of vapour evaporated from the liquid”should be construed in a broad sense as covering not only active steps,such as application of reduced or increased pressure, but also the mereensuring that such vapour can be vented.

[0036] The process is preferably performed as a continuous operation,wherein

[0037] said lactose-containing aqueous liquid is introduced into one endof an elongated essentially horizontal zone;

[0038] the heating in measure (i) is carried out by supplying a heatingmedium to a jacket surrounding at least a portion of said zone;

[0039] said vapour evaporated from the liquid is in measure (ii) removedby purging said zone by passing an air flow therethrough;

[0040] the agitation in measure (iii) is performed by rotating anessentially horizontal shaft carrying agitation means to agitate thetotal volume of liquid present in said zone; and

[0041] said slurry is recovered from the end of the elongated zoneopposite to said one end.

[0042] In a typical embodiment of the process, the lactose-containingaqueous liquid used as starting material is selected from the groupconsisting of concentrates of permeate formed by ultrafiltration ofsweet or acid whey or milk, concentrates of whey or of demineralizedwhey, mother-liquor from lactose recovery, and lactose slush.

[0043] In the above-described continuous operation, the heating ispreferably performed by supplying steam to the jacket mentioned. If theliquid to be treated is permeate concentrate coming directly from aconventional evaporator its content of dry solids can typically be55-60% and its temperature typically 65° C.

[0044] By passage through said zone, the amount of total dry solidsincreases to e.g. 87% and the amount of crystallized α-lactosemonohydrate to 85% of the lactose.

[0045] Due to the previous intensive agitation of this pseudo-plasticand thixotropic mass, by the recovering from said zone it forms a gruel,which is still transportable, e.g. pumpable.

[0046] The above-mentioned cooling and optional disintegration andfurther drying can for instance be accomplished by passing said gruelthrough an auger apparatus having cooling surfaces, and subsequentlythrough a disintegrating and drying device.

[0047] However, the cooling may be a natural cooling or a forcedcooling.

[0048] The invention further comprises a plant suitable for carrying outthe above-described process.

[0049] Thus, in this aspect the invention deals with a plant forconverting an aqueous lactose-containing liquid into free-flowingparticles comprising:

[0050] (a) a first device having (i) an elongated cylindrical horizontalhousing of essentially circular vertical cross-section, (ii) an inletfor lactose-containing liquid at one end of said housing, (iii) arotatable horizontal shaft axially in said housing, (iv) paddlesradially extending from the shaft towards the cylindrical wall of thehousing, (v) a heating jacket surrounding at least partially saidhousing, (vi) openings at each end of the housing for removing vapoursand, (vii) an outlet in the housing for lactose-containing gruel nearthe end opposite to said one end,

[0051] (b) a cooling device connected to said outlet end designed as anauger having at least one screw end being provided with a jacket and/orhollow screw shaft(s) for receiving a cooling medium, and

[0052] (c) a disintegrating and drying device connected to said coolingdevice.

[0053] The device defined under (a) can be rather similar to anapparatus disclosed in U.S. Pat. No. 3,425,135. In said disclosure, theapparatus is described as an apparatus for processing solids and all theexamples illustrate drying of such.

[0054] The residence time in this apparatus can be adjusted by settingthe padres in a backward pitch. Thereby sufficient time for an almostcomplete crystallization can be obtained when performing the process ofthe present invention. An apparatus of similar construction is disclosedin U.S. Pat. No. 5,271,163, wherein it is described as suitable forflowable materials. However, this last-mentioned apparatus is specificby having means for introduced gas through nozzles in the rotating partswith a view of spreading the material to be treated over the walls ofthe housing. No applications akin to lactose crystallization seem to bementioned in said US patent.

[0055] The process and the plant according to the invention are furtherelucidated below by reference to the drawing.

BRIEF DESCRIPTION OF THE DRAWING

[0056]FIG. 1 schematically depicts an embodiment of a plant according tothe invention suitable for performing the process of the invention.

[0057]FIG. 2 schematically shows an alternative suitable embodiment of adetail of the plant of FIG. 1.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0058] In FIG. 1 a device wherein the starting liquid is heated andagitated and evaporated and crystallized is generally indicated 1.

[0059] This device comprises a cylindrical elongated housing 2 andtherein a shaft 3 rotatable by means of a motor 4. On said shaft paddles5 extend outwardly to a point adjacent to the wall of the housing butthe paddles do not contact said housing.

[0060] In the embodiment shown, the shaft 3 is axially centred to haveall paddles at the same distance from the wall of the housing 2.

[0061] The housing is at least partially surrounded by a heating jacket6, which in the shown embodiment is divided into two sections.

[0062] To said jacket, a heating medium, such as hot water or steam,e.g. at a temperature of 50-150° C. is introduced through inlets 7 andcondensed water and any surplus of steam is withdrawn from outlets 8.

[0063] In contrast to what is shown in the drawing, the inner wall ofthe jacket will preferably be in line with the cylindrical wall of thehousing 2.

[0064] The housing has an inlet opening 9 for the concentrate used asstarting material and an outlet 10 for the α-lactose monohydratecrystals containing gruel resulting from the treatment in the device 1.

[0065] The device further has an entrance 11 and an exit 12 connected toa fan 13 to provide a draft whereby filtered atmospheric air enters theinterior of the housing through 11, passes through and purges thehousing and is exhausted through 12 and 13.

[0066] Thus, in the embodiment depicted the water vapour formed by theevaporation in the housing is removed by purging with air at ambienttemperature. However, it would be possible to supplement or substitutethe heating by means of the jacket 6 by drawing a stream of moderatelyheated air through the housing.

[0067] When operating said device and performing the process of theinvention, the starting material liquid is introduced through inlet 9and the shaft 3 with paddles 5 is rotated by means of the motor 4.

[0068] Within the housing 2, the liquid is subjected to a high shearagitation by the paddles and is thrown outwards due to the centrifugalforce and thus forms an agitated ring along the inner surface of thehousing and the heating jacket 7. Thereby an efficient heat transfer isobtained causing a rise of temperature and resulting evaporation of theliquid. Also the shaft 3 may be heated.

[0069] The paddles are spaced on the rotor so that all places of theheated inner wall of the housing are affected periodically when thepaddles are rotating.

[0070] In the embodiment of FIG. 2, paddles 5′ on shaft 3′ are arrangedin mutually displaced rows to ensure a uniform affection of all areas ofthe heated inner wall of the housing.

[0071] The shear rate at the most affected locations is given by thepaddle peripheral speed, which is typically 5-15 m/s, divided by thedistance from the paddle tip to the inner wall of the housing, typicallybeing 5-25 mm. The shear rate provided by the paddles is typically200-3500 s⁻¹, preferably 300-3000 s⁻¹, and more preferably 400-2500 s⁻¹.

[0072] The position of the paddles 5′ the rate of introducing startingmaterial through 9 and possibly a derivation of the position of thehousing away from the horizontal can be used to obtain a desiredresidence time within the housing.

[0073] Guiding experiments indicate that residence times of 2-15 mm willbe sufficient.

[0074] When reaching the outlet 10 as a hot gruel, the crystallizationis substantially completed and the portion of α-lactose monohydratecrystals may typically be 80-90% of the lactose and the free moisturecontent 8-18%.

[0075] At that stage, the gruel is transportable/pumpable but if asample thereof is allowed to cool without agitation, the effects due topseudo-plasticity and thixotropicity disappear and the sample becomeshard.

[0076] Therefore, the gruel recovered through 10 is directed to adevices which in the depicted embodiment is a double-screw anger 14having a cooling jacket 15. Preferably, said device also has hollowscrew shafts (not shown) for introducing cooling medium.

[0077] In the auger 14, the material being treated cools down to atemperature of e.g. 20-40° C. and mainly due to thermo-plastic reasonsthe viscosity increases substantially and a thick non-pumpable mass isformed.

[0078] However, the cooling device may be built together with the device1 as a section having a cooling jacket instead of a heating jacket and,possibly, separate agitation means.

[0079] In the shown embodiment, this mass is transferred to adisintegrating and drying device, which may be a fluidized bed apparatus16 having a gas distribution plate 17 and a stirring means 18 fordisintegration of the mass received from 14. Drying air is introduced byfan 20 through a heater 19 and fluidizes and dries the material in a bedformed above the plate 17. An outlet 21 acts as a weir and the productis recovered through said outlet.

[0080] Optionally, crystallized final or intermediate products may berecycled to the inlet 9 of the device 1, or to the inlet of the coolingdevice whether this is a section of device 1 having a cooling jacket, oris a separate cooling device.

[0081] Materials prepared as described above have been subjected tovarious tests showing their superiority over conventional products,especially their non-caking and tree-flowing properties.

[0082] To further elucidate an embodiment of the invention, thefollowing non-limiting example is presented.

EXAMPLE

[0083] The pilot plant used in this example comprised a small version ofa device marketed under the name ROSINAIRE™. This is an apparatusoriginally designed for drying fibres and other solids. In principle, itis constructed as the device 1 in the drawing.

[0084] The horizontal housing of the device had an inside diameter of254 mm, and the distance from the paddle tips to the interior wall ofthe housing was approximately 6 mm.

[0085] The shaft with paddles was rotated at a velocity corresponding toa peripheral speed of the paddles of approximately 9 m/s, correspondingto a shear rate of 1,500 s⁻¹.

[0086] In commercial applications, it is foreseen to use substantiallylarger devices having inside diameters of 1-2 m and shear rates of300-800 s⁻¹.

[0087] The starting material was a demineralized permeate concentratehaving a tree-moisture content of 45% and a temperature of 65° C.

[0088] This starting material was supplied through the inlet 9 at a rateof 130 kg/h. Steam was introduced through 7 at a temperature of 109° C.and ambient air was drawn in through 11 at a temperature ofapproximately 30° C.

[0089] This air was removed together with water vapour evaporated fromthe permeate concentrate through exit 12 at a temperature of 64° C.

[0090] The position of the paddles and the amount of starting materialintroduced into the device was adjusted to obtain a residence timetherein of approximately 4 min.

[0091] During the passage through the device, permeate concentrate wassubjected to a heating and simultaneously to a high shear rate (1,500s⁻¹ as explained above), and the air flow maintained through the deviceprovided an efficient means for removing water vapour from theextensively agitated liquid, thereby obtaining an efficient evaporationat temperatures substantially below the boiling temperature of saidconcentrate.

[0092] This evaporation caused an increase of the lactose concentrationresulting in crystal formation and growth. This formation and growth ofcrystals were promoted by the viscosity decrease of the slurry, forwhich reason only a moderate super-saturation was experienced. This isin contrast to other processes involving evaporation of aqueous lactosesolutions, in which no specific measures are taken to utilizepseudo-plastical and thixotropical capabilities to decrease viscosity byapplying a high shear rate.

[0093] When reaching the outlet 10, the amount of free-moisture in thecrystal slurry had been reduced to 13% and the temperature thereof was64° C. 85% of the lactose was at that stage present as crystallineα-lactose monohydrate. The amount of gruel withdrawn per hour through 10was 83 kg/h having 87% solids.

[0094] This material was transferred to the cooling device 14.

[0095] To imitate cooling by means of a jacket 15, cooling was obtainedby introducing liquid nitrogen into the device 14 together with thematerial from 10. Thereby the latter was cooled to a temperature of 30°C., and it was transferred to a stirred fluidized bed apparatus fordrying to reduce the free-moisture to 2%.

[0096] An amount of 75 kg/h of this final product was recovered as afree-flowing, non-hygroscopic particulate material.

1. A process for evaporative concentration and crystallization of aviscous lactose-containing aqueous liquid, comprising; (a) introducingthe lactose-containing aqueous liquid having a dry solids content of40-75% by weight into a zone and therein subjecting it to the followingsimultaneous measures; (i) progressive heating at a temperature above40° C. but below the maximum temperature for crystallization ofα-lactose monohydrate from the liquid; (ii) removal of vapour evaporatedfrom the liquid; and (iii) mechanical agitation able to provide acrystallization promoting decrease of the viscosity of the liquid withcrystals formed and suspended therein; to progressively concentrate andsimultaneously crystallize the agitated liquid, and (b) recovering theresulting slurry from said zone at a total solids content above 75% forcooling and optional disintegration and further drying.
 2. The processof claim 1 performed as a continuous operation, wherein saidlactose-containing aqueous liquid is introduced into one end of anelongated essentially horizontal zone; the heating in measure (i) iscarried out by supplying a heating medium to a jacket surrounding atleast a portion of said zone; said vapour evaporated from the liquid isin measure (ii) removed by purging said zone by passing an air flowtherethrough; the agitation in measure (iii) is performed by rotating anessentially horizontal shaft carrying agitation means to agitate thetotal volume of liquid present in said zone; and recovering said slurryfrom the end of the elongated zone opposite to said one end.
 3. Theprocess of anyone of claims 1 or 2, wherein said lactose-containingaqueous liquid used as starting material is selected from the groupconsisting of concentrates of permeate formed by ultra filtration ofsweet or acid whey or milk, concentrates of whey or of demineralizedwhey, and mother-liquor from lactose recovery, and lactose slush.
 4. Theprocess of anyone of claims 1 or 2, wherein the heating in measure (i)is caused to increase the temperature of the liquid to 50-90° C.
 5. Theprocess of claim 1, wherein the removal of vapour in step (ii) isaccomplished by maintaining a reduced pressure in said zone.
 6. Theprocess of anyone of claims 1 or 2, wherein the agitation of measure(iii) is performed at an intensity to provide a shear rate of 200-3,500s⁻¹ in the liquid.
 7. The process of anyone of claims 1 or 2, whereinthe agitation of measure (iii) is performed at an intensity to provide ashear rate of 300-3,000 s⁻¹ in the liquid.
 8. The process of anyone ofclaims 1 or 2, wherein the agitation of measure (iii) is performed at anintensity to provide a shear rate of 400-2,500 s⁻¹ in the liquid.
 9. Theprocess of claim 1 or 2, wherein said cooling and optionaldisintegration and further drying comprise passing said slurry throughan auger apparatus having cooling surfaces and subsequently through adisintegrating and drying device.
 10. A plant for converting an aqueouslactose-containing liquid into free-flowing particles comprising (a) afirst device having (i) an elongated cylindrical horizontal housing ofessentially circular vertical cross-section, (ii) an inlet forlactose-containing liquid at one end of said housing, (iii) a rotatablehorizontal shaft axially in said housing, (iv) paddles radiallyextending from the shaft towards the cylindrical wall of the housing,(v) a heating jacket surrounding at least partially said housing, (vi)openings at each end of the housing for removing vapours and, (vii) anoutlet in the housing for lactose-containing gruel near the end oppositeto said one end, (b) a cooling device connected to said outlet, and (c)a disintegrating and drying device connected to said cooling device. 11.The plant of claim 10, wherein the paddles (iv) are spaced and mutuallydisplaced on said shaft and have such dimensions that the main area ofthe inner wall of the housing surrounded by the heating jacket (v) isperiodically passed by a paddle tip at a distance of 5-25 mm.
 12. Theplant of claim 10, wherein the cooling device (b) is designed as anauger having at least one screw and being provided with a jacket and/orhollow screw shaft(s) for receiving a cooling medium.
 13. The plant ofclaim 10, wherein the disintegrating and drying device is selected froma stirred fluidized bed and a tumbling drying drum.